The invention lies within the technical scope of the production of lenses for spectacles through the technology of injection moulding lenses onto polarising optical filters.
Polarising lenses are widely used by opticians because of their functional advantages and the benefits which they confer. Lenses with a polarising filter in fact filter out some electromagnetic radiation, allowing only rays originating from direct sources to pass, neutralising reflected light, and in particular blocking out horizontal light rays, that is those which are likely to cause dazzle. Polarising filters also attenuate UV-A and UV-B radiation. Spectacles with polarising lenses neutralising reflected light and the dazzling effect of reflection therefore provide clear relaxed vision, bringing about visual well-being and improving visual performance, as well as also allowing colours and contrasts to be better perceived.
A known method for manufacturing polarising lenses provides for the preparation of an optical filter structure, also known in the pertinent technical field by the term “wafer”, on which a lens structure is co-moulded through injection moulding in a suitable mould.
A difficult stage in the abovementioned process is that relating to preparation of the wafer before insertion in the mould, a stage which requires a dimensional and quality check, the possible removal of protective films and cleaning of the surfaces of the filter, the degree of cleaning having a substantial influence on the final quality that can be achieved through injection co-moulding of the lens.
In the context of conventional processes for the manufacture of polarising wafers it is found that these processes do not always provide an optimum shape of optical filter as regards use for the subsequent production of polarising lenses.
In fact because of limits with the materials constituting the wafer and/or limits on the methods of manufacturing/shaping of the wafer, it can happen that they have a curvature which is not precisely spherical. For example a recurrent defect is that of a “toric” shape, in which the optical filter has two different radii of curvature corresponding to the two main axes (longitudinal and transverse), instead of having a constant radius along all axes.
A defect in the shape of the wafer, even if minor, can give rise to defects or even “rejects” in the polarised lenses produced from them.
In fact, on the assumption that it is desired to manufacture a lens having a spherical surface, if the wafer does not in itself have a perfectly spherical surface it is unable to make a perfect “match” with the cavity in the mould for injection of the lens. The wafer must therefore be to some extent forced into the cavity of the mould to ensure that its outer surface is in as close contact as possible with the surface of the cavity.
Two alternative disadvantages may typically arise in this situation:
a) the wafer may break during the co-injection process as a result of the injection process conditions on the material of the deformed wafer, which experiences tension in this way;
b) if the wafer does not break, it may happen that once the lens has been obtained the insides of the wafer will “remember” its original shape, and will tend to recover it to some extent, causing deformation of the polarising lens.